Master cylinder for hydraulic brakes



March 28, 1939. o. RASMUSSEN MASTER CYLINDER FOR HYDRAULIC BRAKES Filed June 1., 1936 Patented Mar. 28, 1939 UNITED STATES MASTER CYLINDER FOB. HYDRAULIC BRAKES Olaf Rasmussen, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application June 1, 1936, Serial No. 82,702

1 Claim.

This invention relates to a cylinder and piston arrangement useful elsewhere but designed especially for the master cylinder of a hydraulic brake system for vehicles.

An object of the invention is to provide an arrangement whereby the tendency to draw air into the system is avoided.

Another object is to provide a substantially free return of the piston of the master cylinder after brake application.

As a further object the invention makes possible a positive connection between the piston and its operating rod.

Simplicity in construction and economy in invention aims to accomplish.

In the accompanying drawing,

Fig. 1 is a diagrammatic plan view of the hydraulic brake system as applied to a vehicle.

Fig. '2 is a view in elevation as seen from line 2-2 of Fig. 1.

Fig. 3 is vertical section through the master cylinder.

Fig. 4 is a section on line 4-4 of Fig. 3.

Fig. 5 is a vertical section of a modified form.

Fig. 6 is a section on line 66 of Fig. 5.

In the drawing, numeral H is used to designate the wheels of a vehicle. Associated with each wheel is a drum l3 closed by a cover l5. Within the drum enclosure is a suitable brake mechanism (not shown) to be forced into engagement with the drum by the conventional wheel cylinder assembly (also not'shown) of a hydraulic brake applying means. The applying means includes, in addition to the wheel cylinders, a master cylinder I9 and a conduit I! leading therefrom to the several wheel cylinders. As is usual, the brake is applied by depressing apedal 2| hinged at 23 to a bracket 25 secured at 21 to a cross frame bar 29 of the chassis. This bracket also carries the master cylinder as shown best in Fig. 2. The novelty herein is to be found 4 in the master cylinder Hi. This cylinder has at its lower end a cap 3| into which is threaded the 45 fitting 33 at the end of conduit l'l'. At its upper end the cylinder I9 is enlarged to form a reservoir 35 closed by a cover 31 secured by fastening means 39. A piston rod 4| reciprocates through cover 31. At its upper end the rod has an eye 43 for pivotal connection at 45 with a link 46 hinged to the pedal by pin 43. Depression of the pedalpushes the rod 4| downwardly through the reservoir. A protecting boot 4! of accordion form is secured at one end to the cover 31 and at its other end to the piston rod.

manufacture are among other objects which the At the junction of the cylinder l9 and reservoir 35 is a disc 49 held against a shoulder 5| by a ring 53. The piston rod 4| passes through this disc 49 with a clearance at 54. The rod 4| is formed 7 with an inner piston 55 shaped at its lower face 5 to form a seat 51. Surrounding inner piston 55 is an outer or main piston 59 slidable in cylinder l9. This piston 59 has a hole therethrough for the passage of rod 4| Around the rod there are axial passages 6| in thepiston. On the lower 10 face of piston 59 is a sealing cup 63 of rubber or the like. This rubber seal 63 and a metal disc 65 have openings of a diameter in excess of that of rod 4|. At the end of the rod 4| is a disc 6! held by a member 69 threaded into the end of the rod and a spring II is positioned between discs 65 and 61.

In the retracted position of the pistons when both piston 55 and the skirt part I3 of piston 59 engage the stop disc 49, it will be seen that the seat 51 of piston 55 is spaced upwardly from a compressible seal 15 located in an annular recess at the top of the body part of piston 59. With the two pistons in this relative position, there is a free passage between the reservoir and the space beneath piston 59.

The operation of the device is as follows: It may be assumed that a pedal releasing springsuch a spring as 11 for examplehas withdrawn the pistons to the positions shown by Fig. 3 where both the inner piston 55 and the skirt of outer piston 59 are stopped by disc 49. With the-parts in that position fluid may flow under the influence of any pressure differential in either direction between the reservoir and the master cylinder by way of the passages 6|, the raised valve seat 51, the space between the pistons and opening 54. Depression of the pedal brings seat 5l into sealing contact with seal 15 closing the passage and thereafter both pistons move down together forcing fluid through the conduits to the wheel cylinders, whereupon the wheel cylinder pistons move and expand the shoes into contact with the drums. When the brakes are to be released the pedal is restored by spring 'l'l. It will be appreciated that the brake shoe retracting springs may unable to restore the pistons of the wheel cyli ders and effect the reverse flow of fluid as rapidly as the pedal spring 11 seeks to restore the master cylinder piston. As a result 50 there is a tendency to develop at least a temporary ders and in some installations expedients have 55 been resorted to to prevent it. In the present case the first movement of piston rod 4| lifts the seat 51 from seat I5 at whatevenposition in the cylinder I! the parts may be. Further upward movement of the two pistons jointly occurs without appreciably resistance or development of low pressure because of the open passage between the reservoir and the cylinder l9. During this return movement fluid may flow from the reservoir around the inner piston and through passages 8| to the cylinder l9, thus supplementing any insuffleiency in the return flow from the conduit to the master cylinder.

' master cylinder reach their fully retracted position, it is quite likely that there will be a-reverse flow through the space between the pistons and into the reservoir as the brake shoe retracting springs more leisurely eifect the return flow of fluid. With this simple construction, therefore, there is no danger of introducing air into the sys- -tem, and the master cylinder parts may be restored to release position with no appreciable delay.

In Figs. 5 and 6 is shown a modified form. The reservoir 35 is formed integral with the cylinder I! as before. The ring llll holds the stop disc ill against which the inner piston I05 and the outer piston ill! contact. The lower end of piston rod I 09, corresponding to rod ll of Fig. 3, is hinged by a pin III to the forked end oi the inner piston. The inner piston has a pin I I! the ends of which move in slots H5 in thewall of the outer piston I01. Securing means H1 attach a seal N9 of rubber or the like to the lower end of inner piston I05. This seal is adapted to engage a seat ill and an opening I23 at the head or lower end of the outer piston. A suitable sealing cup I25 is seated in a groove of the outer When the pistons 01 the' piston around the opening I23, the seal to prevent the passage of fluid when the piston moves downwardly. It will be clear from Fig. 5 that when the seal H9 is raised from the seat 12! fluid may pass in either direction between the' 5 cylinder 19 and the reservoir 35, the opening in stop disc I03, the spaces between the walls of the pistons and the opening I23 serving for the passage. Limited movement between the two pistons is afforded by pin I I3 and slots H5 whereby the inner piston may be raised and seal H9 lifted from the seat before the pin ill engages the end of its slot to cause a joint upward movement of the two pistons. The operation of this modification will be understood, it being substan-sli tially like the operation of the from shown by Figs. 1 to 4.

I claim: v

A master cylinder for a hydraulic, brake system, a main piston movable in said cylinder, 2. piston rod mounted to reciprocate axially relative to said main piston. an inner-piston positively connected to said rod, cooperating seal and seat members carried by said main piston and inner piston,'means whereby said rod constitutes the 26 sole means for moving said inner piston and is adapted'to move it in both directions, movement in one direction acting to cause spaced relation between the seal and the seat, movement of the rod in the opposite direction operable to effect 30 contact of said seat and seal, both piston and inner pistonthereafter moving together, said means whereby the rod moves the main piston when the seat and. seal are spaced comprisin a compressible spring between the main piston and the end of the rod.

OLAF RASMUSSEN. 

